As background, optical fiber attenuators are used for optical signal-power reduction mainly in short distance telecommunication links and in long distance trunk lines with signal repeaters having a fixed amplification. Whenever the optical signal power is higher than the range of the detectors, the optical signal needs to be lowered. Lowering the signal is accomplished by an attenuator.
While there are a variety of different attenuators, the present invention relates to a mechanical attenuator. Fiber can be attenuated mechanically in several ways. One method of attenuation is attained through bending or bundling of the fiber as illustrated in Smith U.S. Pat. No. 5,677,977 and Slaney et al. U.S. Pat. No. 5,684,912, thereby distorting the fiber and causing optical loss through the fibers. Another method of attenuation involves distorting the diameter of the fiber by heating the fiber and then compressing or pulling the cable to change its diameter, as disclosed in Takahashi et al. U.S. Pat. No. 5,321,790. Yet another method of attenuation involves axially aligning two fibers with a gap between the ends of the fibers. Air between the gap can then be used to attenuate, or the gap can be filled with a density filter or film type optical attenuator as disclosed in Serafini et al. U.S. Pat. Nos. 5,706,379 and 5,805,760.
The mechanical attenuator of the present invention is an attenuator that utilizes an air gap between the end of two fibers to attenuate the transmission signal. Typically, an attenuator having an air gap between the fiber attenuates in one of three ways. The first type of attenuation is transverse attenuation and occurs by offsetting the axes of the fiber such that the axes of the fiber remain parallel to one another. The second type of attenuation is angular attenuation, which occurs by varying the angle of the end of the fibers relative to one another. Finally, the third type of attenuation is longitudinal attenuation, which is achieved by varying the distance of the gap between the ends of two axially aligned fibers.
In the prior art, those attenuators that teach variable attenuation through angular attenuation typically angle the ends of the axially aligned cables at a fixed distance apart, as shown in FIGS. 1 and 2, and rotate one fiber relative to the other to vary the amount of attenuation. When one end fiber is rotated relative to the other, the opposing tangent planes of the ends of the fibers move from being generally parallel to one another to being offset by as much as 20 to 40 degrees. Thus, the attenuation can be increased and decreased by simply turning one ferrule relative to another.
The prior art attenuators that teach angular attenuation also teach the rotation between the fibers to be adjustable between 0 to 180 degrees. FIG. 1 illustrates a prior art attenuator in its resting position, with no rotation between the fibers. In contrast, FIG. 2 illustrates a prior art attenuator with 180 degree rotation between the fibers. Rotation between the fibers allows for an increase in attenuation as the fiber is rotated in one direction and a decrease in attenuation when the fiber is rotated in the opposing direction. By only allowing for an increase in attenuation in one direction, a field technician must, for each attenuator he encounters, determine which direction (clockwise or counterclockwise) provides an increase in attenuation and which provides a decrease.
To improve upon the prior art, the present invention provides for the 360 degree rotation of one fiber end 28 relative to the other (FIG. 3). Unrestricted rotation of the ferrule 28 allows the optical signal to be both increased and decreased by rotating the first housing 12 in only one direction. Additionally, to achieve further variation in the amount of attenuation, in one embodiment of the present invention, the predetermined distance between the ferrules 28 can be varied by changing the interior alignment of the first housing member relative to the second housing member.